title: Inositol tetrakisphosphate-induced sequestration of Ca2+ replenishes an intracellular pool sensitive to inositol trisphosphate.
abstract: In a permeable neoplastic rat liver epithelial (261B) cell system, inositol 1,3,4,5-tetrakisphosphate--Ins(1,3,4,5)P4--induces sequestration of Ca2+ released by inositol 2,4,5-trisphosphate--Ins(2,4,5)P3; a non-metabolized inositol trisphosphate (InsP3) isomer--and Ca2+ added exogenously in the form of CaCl2. Studies were performed to identify the Ca2+ pool filled after Ins(1,3,4,5)P4 treatment. Both Ins(2,4,5)P3 and inositol 1,4,5-trisphosphate--Ins(1,4,5)P3--dose-dependently release Ca2+ from permeable 261B cells--Ins(1,4,5)P3 having a threefold greater potency--but differ in that Ca2+ released by Ins(1,4,5)P3 is readily sequestered, while the Ca2+ released by Ins(2,4,5)P3 is not. Maximal release of Ca2+ by 6 microM Ins(2,4,5)P3 blocked the action of Ins(1,4,5)P3, demonstrating that these two isomers influence the same intracellular Ca2+ pool through a shared membrane receptor. Addition of 2 microM Ins(2,4,5)P3 to discharge partially the Ca2+ pool reduced the amount of Ca2+ released by a maximal dose of Ins(1,4,5)P3 (2 microM). Ins(1,3,4,5)P4 combined with Ins(2,4,5)P3 produced a Ca2+ release and sequestration response, which replenished the InsP3-sensitive pool as indicated by a recovery of full Ca2+ release by 2 microM Ins(1,4,5)P3. Induction of Ca2+ sequestration by Ins(1,3,4,5)P4 occurred dose-dependently, with a half-maximal response elicited at a dose of 0.9 microM. Further studies of the effect of Ins(1,3,4,5)P4 apart from the influence of Ins(2,4,5)P3 using a model in which the Ca2+ levels are raised by an exogenous addition of CaCl2 showed that Ins(1,4,5)P3 released twice the amount of Ca2+ from the storage pool following Ins(1,3,4,5)P4-induced Ca2+ sequestration. These results demonstrate that the Ca2+ uptake induced by Ins(1,3,4,5)P4 preferentially replenishes the intracellular Ca2+ storage sites regulated by Ins(1,4,5)P3 and Ins(2,4,5)P3.
authors.affiliation: Cancer Research Center of Hawaii, University of Hawaii, Honolulu 96813.
authors.forename: T D
authors.initials: TD
authors.lastname: Hill
authors.forename: A L
authors.initials: AL
authors.lastname: Boynton
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D004848:Epithelium / Q000378:metabolism / Q000473:pathology; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D007295:Inositol Phosphates / Q000494:pharmacology*; D008113:Liver Neoplasms / Q000378:metabolism / Q000473:pathology*; D051381:Rats; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000378:metabolism / Q000473:pathology
chemical_list: D007295:Inositol Phosphates; C081655:inositol-1,3,4,5-tetrakisphosphate; D015544:Inositol 1,4,5-Trisphosphate; C062616:inositol 2,4,5-trisphosphate; D002118:Calcium